The electrolytic production of chlorine and caustic soda by the electrolysis of brine, viz., an aqueous media containing sodium chloride, has been well-known for many years. Electrolytic cells for chlor-alkali synthesis have historically been of three types, viz., diaphragm cells, mercury cells and membrane cells.
Diaphragm cells use a hydraulically-permeable diaphragm, usually made of asbestos, which is customarily vacuum deposited onto a foraminous cathode substrate, e.g., steel. Diaphragm chlor-alkali cells have been widely commercialized. Such diaphragm cells, employing permeable diaphragms, produce sodium chloride-containing sodium hydroxide catholytes because sodium chloride passes through the diaphragm from the anolyte to the catholyte. Such sodium chloride-containing caustic generally requires a de-salting procedure to obtain a low salt caustic for industrial purposes. Recently, the chlor-alkali industry has focused much of its attention on developing membrane cells to produce low salt-free caustic soda in order to improve quality and avoid costly de-salting processes. Membranes have been developed for that purpose which are substantially hydraulically-impermeable, but which will permit hydrated Na.sup.+ ions to be transported from the anolyte portion to the catholyte portion, while substantially preventing transport of Cl.sup.- ions. Such cells are conventionally operated by flowing a brine solution into the anolyte portion and by providing salt-free water to the catholyte portion to serve as the caustic medium. Hydrogen is evolved from the cathode, and chlorine from the anode, regardless of whether a membrane cell or a diaphragm cell is employed.
The media present in the anolyte and catholyte compartments of chlor-alkali cells are exceedingly corrosive. Particularly, there exists a need for a gasket which can withstand the corrosive atmosphere of the anode compartment in as much as the brine, wet chlorine gas and hypochlorites at 180.degree. to 220.degree. F. and pressures from atmospheric to about 50 psig tend to corrode the rubber gasket and result in escalating cell voltage and declining current efficiency.
Prior to the present invention, the gasketing used in chlor-alkali cells was chiefly rubber, including synthetic elastomers. While such rubber or elastomer material worked well initially, after extended periods of use, problems developed within the chlor-alkali cells which were traceable back to the rubber gasketing. For example, contaminants produced by the corrosion of gasketing material caused the operating voltage to increase. Since the production of chlorine and alkali in these cells is very cost related to the cost of electric power required to operate such cells, the chemical resistance of the gaskets to the anolyte became of great importance. By and large, the copolymer or terpolymer elastomer materials seem to exhibit greater resistance to chemical degradation than the rubber materials, e.g., neoprene. For example, ethylene/propylene diene (EPDM) elastomeric rubber gaskets show greater resistance to anolyte degradation than gaskets made of neoprene. However, even these EPDM gaskets were not able to sufficiently resist degradation on the anolyte side of the electrolytic cell. Because of this, the EPDM elastomer gasket life is not projected to any longer extent rubber gaskets, e.g., neoprene.
In an effort to overcome these problems, attempts were made to utilize various forms of polytetrafluoroethylene (PTFE) to serve as a chemically resistant gasket material for the anolyte compartment of chlor-alkali cells. The use of PTFE gaskets, per se, is much more expensive than rubber and elastomer materials and has not proved entirely successful. Moreover, it is difficult to reuse the PTFE material once a cell has been shut down due to other considerations, viz., other than failure or chemical degradation of the PTFE gasket. One such PTFE gasket material which has been utilized in chlor-alkali cells on an experimental basis by the present inventor is a material commercially available in the form of a continuous, low density (like a foam) cord from W. L. Gore and Associates Inc. and is sold as "GORE-TEX." Use of the PTFE gasket material "GORE-TEX" did overcome the chemical attack of the corrosive anolyte; but the cells in which such PTFE gasket was utilized exhibited scortching of the membrane which was observed to lay up against the anode gasket surface. The scortching was apparently due to current leakage through the gasket. Moreover, the use of PTFE did not appear to obtain as tight a seal as was obtainable with the rubber, e.g., neoprene or elastomer, e.g., EDPM, gaskets.
The combination of advantages of (1) resistance to corrosion due to chemical degradation by the anolyte (2) while maintaining low cell voltages and (3) high current efficiency necessary for economic production of chlorine and caustic soda and (4) longer useful gasket life are obtainable in accordance with this invention by the use of a multi-layer gasket comprising an outboard layer of a material having a combination of a Type A Shore Scale Durometer Hardness (ASTM Specification No. D-2240-75) ranging from about 40 to about 70 and a Compression Set (ASTM Specification D-395-69-Method A) of zero plus to about 40 percent in combination with an inboard barrier layer of a material which is corrosion-resistant, noncontaminating and stable upon contact with chlor-alkali cell anolyte wherein said barrier layer is positioned inboard on the anolyte wetted side of the anode gasket. Specific preferred embodiments of this invention enable the use of comparatively small amounts of the far more expensive PTFE or equivalent barrier layer material in the form of a barrier rope or strip positioned on the membrane-facing side of the anode frame and located along the periphery thereof between the rubber or elastomer gasket layer and the anolyte. The rubber or elastomer gasket layer can be in contact with the PTFE barrier layer or it can be separated therefrom (as shown in both figures of the drawing). The advantages obtainable utilizing the multi-layer gasket of this invention are such that no decline in cell performance due to chemical degradation was observed on these gaskets used for the anode compartment after testing in a commercial scale sized chlor-alkali cell for a period of four months. Moreover, the cells in which the multi-layer gaskets of this invention were utilized continued to operate at or near initial operating voltage and current efficiency. Such was not the case for either the neoprene gaskets, per se, the elastomeric EPDM gaskets, per se, or the PTFE gaskets, per se.